In ICPT systems it is sometimes necessary to have transmitting and receiving coils in close proximity to other transmitting and receiving coils, to electrically conductive surfaces, or to sensitive electronics. For example, in the multiple power and data transmission channels commonly employed in wind turbines. It is desirable for each channel to be isolated from the adjacent channel. Conventionally this may be achieved using a conductive plate between channels. It may also be desirable to provide shielding to avoid losses due to surrounding metallic elements.
Time-varying magnetic fields generate eddy currents in conductive materials that act to cancel the applied magnetic fields. The result is that magnetic fields are repelled from the surface of the conductor. Within this document, this effect is referred to as “magnetic shielding” or simply “shielding”.
Solid conductive plates provide good shielding but suffer from overheating for two reasons:
Firstly and most importantly, though a conductive plate shield may be physically thick, at ICPT frequencies, only the surface of the conductive plate will conduct current, due to the skin effect. This means that the effective resistance of the conductive sheet is much higher than would be measured at DC, so the resistive losses in the shield can be high.
Secondly, the regions of the solid conductive plate where the incident magnetic field is strongest will have higher losses than other areas with weaker incident magnetic fields. This is because the eddy currents are higher in the regions with high incident magnetic field.
Existing shields suffer from limited flexibility in that their degree of screening cannot be easily tailored to a specific requirement. Other things being equal, better screening corresponds to higher losses and vice versa, therefore it would be useful to have a method whereby the screening effect could be reduced or increased to the required level (and no further) such that losses are minimized while still achieving the desired level of screening.
It would be desirable to provide a magnetic shield that provides effective and flexible shielding whilst avoiding the heating problems of the prior art or to at least provide the public with a useful choice.